In conventional lasers, a laser cavity establishes standing wave modes that function to select the frequencies at which the laser operates. In many applications, including optical communications utilizing optical fibers, it is advantageous for the laser to provide a single-mode beam of light. For optical fiber communication, the wavelength of this single mode should be within the wavelength range in which the optical fiber has minimal attenuation and/or minimum dispersion. At present, this wavelength is on the order of 1.3 microns.
To be commercially viable, the laser should have low power requirements, high reliability, modest size and reasonable cost. To accomodate 100 Megabaud data rates, the linewidth of this single laser mode should be less than 200 kilohertz using heterodyne detection and phase-shift keying data encoding and the laser frequency should be equally stable. Ring lasers have proven to meet these requirements.
In a ring laser, a travelling mode is utilized in place of the standing modes of conventional lasers. An optical source provides light that travels around a closed path (the "ring") within the laser. A typical ring laser (See, for example, U.S. Pat. No. 3,824,492 by Michael J. Brienza, et al, entitled "Solid State Single Frequency Laser", issued July 16, 1974), includes three or more reflective elements to direct a travelling wave around the ring. Since waves can travel in either direction around this ring, such a ring could support modes travelling in opposite directions around the ring.
To ensure single mode oscillation of the laser, the ring laser design should support only one of these two travelling wave modes. The reason for this is that, if modes in both directions of travel are supported, then these modes will interfere spatially to produce spatial variation of the beam intensity around the ring. There is a periodic pattern of nodes and antinodes separated by 1/4 of a wavelength. Because of gain saturation, the gain is reduced where the optical intensity is maximum. Therefore, the gain is also spatially modulated with the maximum gain regions lying at the nodes of the optical interference pattern. This phenomenon is known as "spatial hole burning". Because the wavelengths of adjacent modes are slightly different, the spatial interference patterns generated by the two modes will not coincide. Therefore, each mode will extract gain from spatial regions not saturated by the adjacent mode. For this reason, bidirectional rings and linear ring resonators tend to oscillate in more than one mode.
Support of only one of these two travelling modes has an additional advantage. Many laser applications result in some of the light emitted from the laser reflecting back into the laser. Such reflected light will destabilize the operation of the laser. In a ring laser, such reflected light is in the direction of the unsupported travelling wave mode and therefore is attenuated before it can significantly affect laser operation.
In a typical discrete component ring laser (see, for example, U.S. Pat. No. 3,824,492 by Brienza et al entitled "Solid State Single Frequency Laser" issued July 16, 1974), the polarization of the travelling wave beam is rotated by a wave plate and is also rotated by a Faraday rotator. For the supported mode, these two rotations cancel so that the polarization is unchange by a complete traversal of the ring. For the unsupported mode, these two rotations add to produce a net rotation around the ring. A polarizer is located within the ring to attenuate the unsupported mode and to transmit substantially all of the supported mode. This selective attenuation assures that only one of these two modes is supported.
In the monolithic unidirectional ring laser presented in U.S. Pat. No. 4,747,111 by Trutna, Jr. et al. entitled "Quali-Planar Monolithic Unidirectional Ring Laser" issued May 24, 1988, a single block of material is shaped to direct the a travelling wave beam around a ring and to rotate the polarization of the beam, thereby avoiding the need for a wave plate. This beam reflects off of four sides of this block. This block is formed out of a material that is selected to lase at the wavelength of this travelling wave beam and is also selected to act as a Faraday rotator in the presence of an applied magnetic field. For one direction around the ring, there is a geometrical polarization rotation induced by the out-of-plane reflections that is cancelled by the Faraday rotation allowing a low loss reflection from the polarizing output mirror. In the other direction, the two rotations add leading to an attenuation of the beam by the lossy polarizer. Unfortunately, this laser is not tunable so that it is limited to applications in which only a single beam frequency is needed.